FIG. 1 shows a pixelated OLED device 100 which serves, for example, as a display in various types of consumer electronic products, including cellular phones, cellular smart phones, personal organizers, pagers, advertising panels, touch screen displays, teleconferencing and multimedia products, virtual reality products, and display kiosks.
The OLED device comprises a functional stack formed on a substrate 102. The functional stack comprises of one or more organic functional layers 104 between two conductive functional layers (106 and 108) which serve as electrodes (anode and cathode). The conductive layers are patterned as desired. For example, the conductive layers can be patterned to form rows of anodes in a first direction and columns of cathodes in a second direction. OLED cells or pixels are located where the cathodes and anodes overlap. Charge carriers are injected through the cathodes and anodes via bond pads 112 for recombination in the organic layers. The recombination of the charge carriers causes the organic layer of the pixels to emit visible radiation. The device is encapsulated with a cap 110, hermetically sealing the cells.
As shown in FIG. 1, t-shaped pillars 114 are used to facilitate patterning of the upper conductive layer. The pillars can also be tapered with the top being wider than the bottom. Tapered or t-shaped pillars are described in, for example, Ext. Abstr. 44th Spring Meeting Japan Society of applied Physics and related Societies, 1997, and U.S. Pat. Nos. 5,962,970, 5,952,037, 5,742,129, or 5,701,055, which are all herein incorporated by reference for all purposes. The pillars are formed on the substrate after the formation of the lower conductive layer 106. Thereafter, the organic layer and conductive layer are deposited. Due to the profile of the pillars, the continuity of the upper conductive layer is disrupted, leaving segments of the conductive layer 108a over the organic layer 104 and segments 108b on top of the pillars.
However, the functional stack is susceptible to damage resulting from exposure to atmospheric constituents like oxygen and moisture that penetrated into the interior of the device. The cathode layer comprises, for example, magnesium (Mg), calcium (Ca), barium (Ba), silver (Ag), aluminium (Al) or a mixture or alloy thereof, which are susceptible to damage caused by exposure to any potentially deleterious substance such as water vapor and oxygen.
Referring to FIG. 1, the edges of the functional stack layers are exposed due to the profile of the pillars 114. Open edges such as 120 of the upper conductive layer and organic layer are especially susceptible to damage caused by water and oxygen and are typically areas which are affected first. The result may be shrinking pixels or dark, non-emitting spots due to the lack of current flow, leading to a reduction in the useful life of the OLED device.
Known methods typically employed to protect the functional stack include hermetically sealing the device and providing a desiccant inside the device to absorb oxygen and moisture that permeates through the sealant. However, residual oxygen and moisture still remaining within the encapsulated device will cause the shrinkage of pixels over time, due to the reaction with oxygen and water, typically starting at the exposed edges of the functional layers.
Alternatively, the upper conductive layer comprises an electron-emitting cathode layer and a protective conductive layer. The electron-emitting layer comprises, for example, Ca, Mg and/or Ba, or a mixture or alloy thereof, which are highly reactive to air and water. The protective layer comprises, for example, more stable materials such as silver (Ag), platinum (Pt), chromium (Cr), gold (Au) and/or aluminum (Al) or a mixture or alloy thereof. The protective conductive layer covers a surface of the electron-emitting layer to protect it from exposure, but does not cover the edges of the cathode layer due to the profile of the pillars. Hence, the edges of the cathode layer are still exposed to residual oxygen and water.
As evidenced from the foregoing discussion, it is desirable to provide a method to effectively pattern electrodes in the fabrication of OLED devices and protect the edges of the functional stack from damage caused by exposure to potentially deleterious substances.